In many switched mode power converters the minimal frequency is not limited below 20 kHz, like for instance in quasi-resonant converters. This means that the converter can generate audible noise, which is not acceptable and which can be perceived by the human ear. In some converters like fly-back converters this can happen when the converter is short-circuited, at start up or when turning off the converter. When a quasi-resonant power converter is driven at high power levels, the frequency with which the converter is switched gets lower. The switching frequency is furthermore lowered when the current through the switch gets high, which switch is normally realized with some kind of transistor circuit, like a FET transistor. There thus exists a problem with the converters of today.
U.S. Pat. No. 6,011,361 describes a buck converter for igniting and operating a high-pressure discharge lamp. Here the maximum off time of the transistor switching the converter can be set preventing operations below 20 kHz. In this document there is no monitoring or direct limitation of the frequency, only limitations for the off time are set. These limitations are set all the time irrespective of if the switching frequency is high or low. The off time gets an upper limit of 36 μs and a lower limit of 5 μs. With regard to audible noise, the switch is turned on if a time limit for the off time is reached. In order for this device to work for reducing audible noise, the load and the input voltage have to be known. Since the circuit is arranged for driving a lamp, which load is known, this works well in this environment. It would however not work properly for a power supply for reducing audible noise, because a power supply has to be able to work with several different types of loads and limitation of the switching period is only to be made when the frequency is actually low and not when the converter is working normally. The document does also not describe limiting of the peak current in the transistor in a discontinuous conduction mode.